user_page_attachments() unnecessarily adds an individual CSS file on every page

Problem/Motivation

When possible, we shouldn't attach CSS or JS files, but asset libraries instead.

user_page_attachments() attaches user.module.css, but that's also part of the user/drupal.user library, so it should attach that instead.

But, there is *nothing* in user.module.css that is generically applicable to all pages, yet it is loaded on all pages. It's only necessary for:

1. the permissions configuration UI (for which only user/drupal.user.permissions is loaded currently, it just happens to be the case that user.module.css is loaded on all pages and hence happens to be correct
2. the account settings form (AccountSettingsForm).
3. user module's automatic enhancements for #type => 'password' element (to show password strength hints)

Proposed resolution

1. Split up the CSS file into a user.admin.css one for point 1, to be compliant with https://www.drupal.org/node/1887922. We'll call this library drupal.user.admin. We then attach that library to AccountSettingsForm. This gets added to the drupal.user.permissions library that already exists.
2. And the rest stays as is.

Now there is no more need to load user.module.css on every page! :)

Remaining tasks

None.

User interface changes

None.

API changes

None.